Implantable bladder sensor

ABSTRACT

An implantable sensor for monitoring the condition of an organ, such as the bladder, of a recipients&#39; body. The sensor ( 100 ) comprises a carrier ( 102 ), such as a tubular housing that is attachable to the outer surface of the organ. An impedance-varying means ( 104 ), such as a saline solution, is carried by the carrier ( 102 ). The impedance-varying means ( 104 ) within the carrier varies as the condition of the organ changes to cause in output state of the sensor ( 100 ). The sensor ( 1000 ) can be used for the purposes of monitoring the filling of the bladder, making bladder volume information to the recipient or another person, and causing an alarm signal to be generated when a preset bladder volume is reached.

FIELD OF THE INVENTION

This invention relates to sensing a parameter of an organ of a person'sbody. More particularly, the invention relates to a device for sensing aparameter of a visceral organ of a person's body.

BACKGROUND TO THE INVENTION

Urinary and faecal incontinence are major health problems, particularlywith an ageing population, for which there is no well-accepted medicaltreatment. Such conditions can affect men and women of all ages and is aparticularly prevalent problem for individuals suffering from spinalcord injury, such as tetraplegics, who may be unable to exercisevolitional control over the detrusor muscle of his or her bladder andother functions such as bowel evacuation.

With the advent of Functional Electrical Stimulation (FES) Systems, suchas that developed by the Applicant and described in International PatentApplication No. PCT/AU03/00044, the contents of which are incorporatedherein by reference, it is possible to control bladder function and,more particularly, incontinence and evacuation of a bladder and otherorgans innervated via the sacral region of a person's spinal cord.

To enable this control to be effected, at least one parameter of theorgan being controlled needs to be monitored. This is particularlyimportant when the organ to be controlled is the bladder which has acapacity to fill and store urine. The ability to easily and effectivelymonitor the capacity of such an organ is therefore important to ensurethat appropriate evacuation of the organ occurs that is dependent uponthe state of the organ.

In this regard, it is a preferred feature of the present invention toprovide a means that assists in monitoring parameters associated withthe visceral organs of a body.

SUMMARY OF THE INVENTION

According to one aspect, the present invention is an implantable sensorfor monitoring the condition of an organ of a recipient's bodyincluding:

a carrier for attachment to an outer surface of the organ; and

an impedance-varying means carried by the carrier, the impedance of theimpedance-varying means varying as the condition of the organ changes tocause a change in output state of the impedance-varying means.

The invention is intended particularly for use in monitoring the volumeof a person's bladder. The carrier may be attached, for example, bybeing sutured, to an outer wall of the detrusor muscle of the bladder bymeans of an appropriate surgical procedure. As the volume of the bladderincreases, the wall of the bladder stretches and changes shape.

The carrier may include an attachment means for attachment to an outersurface or wall of the organ, such as the bladder, so that a change inthe shape of the outer surface or wall causes a change in the impedanceof the impedance-varying means.

The sensor may be implemented in numerous ways. For example, in a firstembodiment of the invention, the carrier may comprise a flexible tube,such as a silicone tube, having closed ends.

The impedance-varying means may comprise a conductive material or aconductive mixture containing a conductive material. In one embodiment,the conductive material may be a conductive fluid, such as a salinesolution, contained in the tube. Distortion of the tube, for example, bystretching and/or displacement of the tube, may cause a change inimpedance, for example, resistive impedance, of the conductive medium.

In this aspect, the impedance change may be enhanced by incorporatingsurface features on an inner wall of the tube that increase theinfluence of the change in the impedance of the conductive medium as aresult of its being distorted.

In another embodiment of the invention, the impedance-varying means mayinclude a displaceable element which is displacably arranged relative tothe carrier, displacement of the element relative to the carrier causingthe change in impedance. The change in impedance may be either a changein resistive impedance or inductive impedance. In other words, linearmotion of the displaceable element relative to the carrier may cause achange in impedance which is monitored as a measure of the change in thecondition of the organ, eg. the volume of the bladder.

In yet another embodiment of the invention, the carrier may be acontainer. The impedance-varying means may be arranged in the container,the impedance-varying means comprising a strip of non-conductivematerial on which a segmented conductive layer is superimposed thereon.

The strip may be divided into segments by transversely extending, spacedslits that cause the segments to separate as the change in the conditionof the organ occurs, eg. as curvature of the detrusor muscle occurs dueto increased volume of the bladder. Such a change in the curvature maycause adjacent segments of the conductive layer, on opposed sides of theslits in the strip, to move apart. As a result, a change, being anincrease, in resistive output of the impedance-varying means may occurto provide an indication of increase in volume of the organ, such as thebladder.

In still a further embodiment of the invention, the carrier may be inthe form of a sealed container in which the impedance-varying means isreceived.

The impedance-varying means may comprise a pair of overlying strips withconductors carried on facing surfaces of the strips, the change incondition of the organ, such as the bladder, resulting in relativedisplacement of the conductors on the strips to cause a change inimpedance. The two strips may be anchored together at one end.

When used on the bladder and the volume of the bladder is low, theconductors of the strips may be aligned and have a low resistive output.As the volume of the bladder increases the strips may follow thecurvature of the wall of the bladder. As a result, the conductors of thestrips may move out of alignment, increasing the resistive output toprovide an indication of increase in bladder volume.

In a further aspect, the present invention is an implantable sensor formonitoring the volume of fluid present in a recipient's bladderincluding:

a carrier for attachment to an outer surface of the bladder; and

an impedance-varying means carried by the carrier;

wherein changes in the outer surface of the bladder as it expands uponreceiving fluid are detected as changes in the impedance of theimpedance-varying means.

In this aspect, changes in the outer surface of the bladder preferablycause the carrier and the impedance-varying means to undergo distortion.

In one embodiment of this aspect, the carrier can be an enclosedflexible housing, with, for example, the impedance-varying means beingcontained within the enclosed flexible housing.

In this aspect, the impedance-varying means can comprise a conductivematerial and distortion of the flexible housing can cause a change inimpedance of the conductive material. This impedance change can beenhanced by incorporating surface features on an inner wall of thehousing which increase the influence of the change in the impedance ofthe conductive material as a result of its being distorted. Theconductive material is preferably a conductive fluid, such as a salinesolution.

In another embodiment of this aspect, the impedance-varying means cancomprise a strip of non-conductive material on which a segmentedconductive layer is superimposed. The strip can be divided into segmentsthat separate as the change in the condition of the bladder occurs.

In yet another embodiment of this aspect, the impedance-varying meanscan comprise a pair of overlying strips with conductors carried onfacing surfaces of the strips, the change in the outer surface of thebladder resulting in relative displacement of the conductors on thestrips to cause a change in impedance. In this embodiment, the twostrips can be anchored together at one end.

In a still further aspect, the present invention is an implantablesensor for detecting variations in shape of an organ of a recipient'sbody comprising:

an elongate flexible housing attachable to an organ of the body;

a conductive material contained within the housing;

at least one electrode for detecting variations in impedance of thesensor;

wherein as the body organ changes shape, the housing also undergoes achange in shape which is detected by the electrode as a variation in theoverall impedance of the conductive material within the housing.

In this aspect, the housing is preferably stretched on an increase involume of the organ thereby reducing the cross-sectional area of thehousing and so increasing the impedance of the conductive materialwithin the housing.

In one embodiment of this aspect, the housing can comprise a series ofchambers in fluid communication with each other for receiving theconductive material, each chamber being defined at at least one end bydiametrically opposed raised formations in the housing and wherein, onstretching of the housing, the opposed raised formations move relativelycloser to each other thereby further increasing the impedance of theconductive material within the housing.

The conductive material of this aspect is preferably a conductive fluid,such as a saline solution.

In this aspect, the organ monitored by the sensor is preferably thebladder.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described by way of example with reference to theaccompanying drawings, in which:

FIG. 1 shows a schematic, sectional side view of an implantable bladdersensor, in accordance with a first embodiment of the invention, formonitoring the condition of a person's bladder;

FIG. 2 a shows a detailed section of part of the sensor of FIG. 1 in arelaxed state;

FIG. 2 b shows the detailed section of the sensor of FIG. 1 in astretched state;

FIG. 3 shows a top view of a variation of the first embodiment of thesensor of FIG. 1;

FIG. 4 shows a schematic, sectional side view of an implantable bladdersensor, in accordance with a second embodiment of the invention, formonitoring the condition of a person's bladder;

FIG. 5 shows a side view of an implantable bladder sensor, in accordancewith a third embodiment of the invention, for monitoring the conditionof a person's bladder;

FIG. 6 shows a side view of the sensor of FIG. 5, after a change in thecondition has occurred;

FIG. 7 shows, on an enlarged scale, part of the sensor of FIG. 6;

FIG. 8 shows a three dimensional, schematic view of an implantablebladder sensor, in accordance with a fourth embodiment of the invention,for monitoring the condition of a person's bladder;

FIG. 9 shows a schematic plan view of the sensor of FIG. 8 when thebladder is in a first state;

FIG. 10 shows a schematic plan view of the sensor of FIG. 8 when thebladder is in a second state; and

FIG. 11 shows a schematic, three-dimensional view of the sensor when thebladder is in its second state.

DETAILED DESCRIPTION OF THE INVENTION

The urinary bladder can be considered as a large, flexible muscularballoon that stores urine produced by the body in the kidneys. As thebladder fills and stores urine, it expands and changes shape in a mannerthat is dependent upon the volume of fluid it stores. In this regard,the shape and size of the bladder can provide a direct indication of theamount of urine present in the bladder, which can be used as anindication of when the bladder should be emptied.

Referring initially to FIG. 1 of the drawings, an implantable bladdersensor, in accordance with a first embodiment of the invention, formonitoring the condition of a person's bladder is illustrated and isdesignated generally by the reference numeral 100. In particular, thesensor 100 is used for monitoring the volume of fluid present in aperson's bladder by monitoring the change in size and/or shape of thebladder. While the person is normally a human, the technique can applyequally to other animals.

The sensor 100 includes a carrier 102 which carries an impedance-varyingmeans 104. The carrier 102 is a tube having closed ends. The tube 102contains the impedance-varying means 104 which is in the form of aconductive fluid, such as saline solution.

An inner wall 106 of the tube 102 carries an impedance enhancingarrangement in the form of a series of diametrically opposed raisedformations 108, thereby forming a series of chambers for receiving theimpedance-varying means 104. The role of these raised formations 108 andthe chambers they form being discussed in more detail below.

The tube 102 has a plurality of pads 110 for securing the tube 102 to awall of the bladder (not shown in this embodiment). Two electrodes 112are positioned at opposite ends of an interior of the tube 102. Theelectrodes 112 are used to detect variations in the impedance of thesaline solution 104. The electrodes 112 are connected via insulatedwires 114 to a sensor/stimulator unit (not shown) which detects andmeasures changes in the impedance within the sensor 100.

As can be seen more clearly in FIGS. 2 a and 2 b, the raised formations108 on the inner wall 106 of the tube 102 serve to enhance (amplify) theeffects of impedance change within the tube 102 of the sensor 100. Theraised formations 108 form a series of open chambers along the tube 102,into which the conductive fluid flows. In use, when the sensor 100 ispreferably sutured to the detrusor muscle of the bladder, as the bladderfills with fluid and changes shape and size, the detrusor muscle flexesand stretches to cause the tube 102 also to flex in a direction shown byarrow Z in FIG. 2 b. In a relaxed state, as is shown in FIG. 2 a, theconductive fluid (saline solution) 104 within the tube 102 is not overlyimpeded, and there is a definite conduction path between neighbouringfluid filled chambers within the tube 102 for conduction to occur asshown by the distance X between opposed raised formations 108.

However, when the bladder begins to fill and the wall of the bladderexpands and flexes, the tube 102 of the sensor 100 is also stretched inthe direction of arrow A. This causes the diameter of the tube 102 toreduce. As a result, the opposed raised formations 108 on the inner wallof the tube 102 are drawn closer together thereby reducing thecross-sectional area of the tube 102 as shown by dimension Y in FIG. 2 bof the drawings. A restriction in the flow path between neighbouringchambers of the conductive fluid (saline solution) 104 results, whichcauses an increase in impedance of the sensor 100. The change inimpedance of the sensor 100 in response to the change in size or shapeof the bladder can be calibrated to provide an indication of the volumeof fluid/urine present in the bladder. Whilst the presence of the raisedformations 108 enhances the effect of impedance change of the sensor100, an impedance change may still be detected in the absence of suchraised formations 108 or by arranging the raised formations in adifferent manner than that described above.

FIG. 3 shows a top view of a variation of the embodiment of the sensor100 of FIG. 1. With reference to FIG. 1 like reference numerals refer tolike parts unless otherwise specified.

In this variation, the tube 102 is substantially hairpin or U-shaped.This allows the electrodes 112 to be arranged alongside each otherwhich, in turn, allows the insulated wires 114 to be packaged togetherfor convenience.

The sensor 100 is, once again, fixed to the wall of the bladder via thepads 110. Filling of the bladder causes the tube 102 to stretch in thedirection of arrows B-B, thereby stretching the walls of the tube 102.The conductive medium 104 within the tube 102 (eg saline solution)experiences greater impedance in the stretched state of the tube 102than in the relaxed state. As a result the change in impedance iscalibrated to indicate the change in size or shape of the bladderrepresentative of the amount of urine/fluid stored in the bladder. Asdescribed above, the impedance change could also be amplified byincorporating the raised formations 108 and creating chambers along thelength of the tube 102 as discussed in relation to FIGS. 2 a and 2 b. Itwill be appreciated that, because the tube 102 has a length dimensionthat is much greater than its diameter, the effects of impedance changearising from any lateral expansion of the tube 102 due to flexing of thedetrusor muscle of the bladder will be masked by the significantlygreater increase in length of the tube 102.

Referring to FIG. 4 of the drawings, an implantable bladder sensor, inaccordance with a second embodiment of the invention, for monitoring thecondition of a person's bladder is illustrated and is designatedgenerally by the reference numeral 10. Once again, the sensor 10 is usedin monitoring the volume of a person's bladder 12.

The sensor 10 includes a carrier 14 with an impedance-varying means 16carried by the carrier 14.

In the embodiment of the invention shown in FIG. 4 of the drawings, thecarrier 14 is a length of conductive silicone. The carrier 14 isattached to a wall 18 of the bladder 12 by, for example, sutures 20.

The impedance-varying means 16 is in the form of a probe 22 having afoot portion 24 which bears against an outer surface 26 of the wall 18of the bladder 12.

When the bladder 12 has a low volume, the wall 18 of the bladder 12 isin a relaxed state and, as a result, the carrier 14 is similarly relaxeddue to very little pressure, if any at all, being imparted thereto bythe probe 22.

As the bladder 12 fills with liquid, the bladder changes size and/orshape and the wall 18 of the bladder 12 adopts a degree of curvature.This causes the probe 22 to be urged outwardly in the direction of arrow28. Movement of the probe 22 in that direction stretches the length ofconductive silicone constituting the carrier 14. This causes a change inresistance in the carrier 14. The change in resistance is indicative ofthe volume of liquid present in the bladder and as such can becalibrated to provide an indication of this volume to aid in determiningwhen the bladder should be evacuated.

Referring to FIGS. 5 to 7 of the drawings, an implantable bladdersensor, in accordance with a third embodiment of the invention, formonitoring the condition of a person's bladder is illustrated and isdesignated generally by the reference numeral 30.

The sensor 30 includes a carrier in the form of a sealed bag 32. Thesealed bag 32 contains a non-conductive material in the form of anon-conductive solution 34. An impedance-varying means 36 is containedwithin the bag 32 and has leads 38 projecting through sealed ends of thebag 32. The ends are sealed in a manner to inhibit escape of thesolution 34 from the interior of the bag 32.

The impedance-varying means 36 comprises a strip 40 of a non-conductivematerial. A side of the strip 40 which, in use, constitutes anoperatively outer side (ie. that side of the strip 40 which is furthestaway from the wall of the bladder) carries a layer of a conductivematerial 42.

As illustrated more clearly in FIGS. 6 and 7 of the drawings, the strip40 has transversely extending slits 44 defined therein to form segments46. The layer 42 of conductive material can extend into the slits 44 asdescribed below.

In use, the bag 32 is attached to the outer surface of a wall of thebladder by way of sutures or the like (not shown in this embodiment).

When the bladder is in a relaxed, empty state, the strip 40 lies in asubstantially planar configuration so that the slits 44 aresubstantially closed. As a result, the conductive layer 42 isapproximately continuous due to adjacent segments 46 abutting againsteach other providing a relatively short conducting path between theleads 38, and as such the sensor 30 has a first, low resistance state.

As the bladder fills and changes size and/or shape, the bag 32, togetherwith the impedance-varying means 36, follows the curvature of the wallof the bladder as shown schematically in FIG. 6 of the drawings. As thestrip 40 curves, the slits 44 open separating the segments 46. Thiscauses an increase in the conductive path of the conductive layer 42, asthe conductive layer extends into the slits 44, and a subsequentincrease in the resistive output of the impedance-varying means 36. Bymonitoring the size and/or shape change of the bladder by monitoring theresistive output of the sensor 30, the volume of liquid present in thebladder can be readily determined to provide an indication of thefilling state of the bladder.

Referring now to FIGS. 8 to 11 of the drawings, a fourth embodiment ofan implantable bladder sensor for monitoring the condition of a person'sbladder is illustrated and is designated generally by the referencenumeral 50.

The sensor 50 includes a sealed bag or pouch 54 which is attached to anouter wall of a bladder 52 of the person's body. The pouch 54 carries animpedance-varying means 56 therein. The impedance-varying means 56 ofthe sensor 50 is shown in greater detail in FIGS. 9 to 11 of thedrawings.

The impedance-varying means 56 comprises a first strip 58 of material onwhich a first set of conductors 60 is mounted. The arrangement of theset of conductors 60 is similar to that of a ladder which has been cutlongitudinally. Thus, the set of conductors 60 includes a longitudinallyextending conductor 62 with a plurality of longitudinally spaced,transversely extending portions 64. The set of conductors 60 is arrangedon an operatively outer surface of the strip 58.

The impedance-varying means 56 comprises a second strip 66 whichoverlies the first strip 58. The second strip 66 carries a second set ofconductors 68 on its surface in contact with the outer surface of thestrip 58. The set of conductors 68 is the mirror image of the set ofconductors 60 and comprises a longitudinally extending conductor 70 anda plurality of transversely extending portions 72.

The strip 58 and 66 are anchored together at one end as illustrated at74.

In use, when the bladder 52 is in a relaxed, empty state, the wall ofthe bladder is substantially flat. As a result, the transverse portions64 of the set of conductors 60 lie in register with the transverseportions 72 of the set of conductors 68, as shown in FIG. 9 of thedrawings. As a result, the impedance-varying means 56 has a lowresistive output.

As the bladder 52 fills and changes size and/or shape, its wallstretches and curves. This results the strips 58 and 66 adopting acurved configuration, as shown in an exaggerated state in FIG. 11 of thedrawings, which causes displacement of the sets of conductors 60 and 68relative to each other as shown in FIG. 10 of the drawings. Due to thetransverse portions 64 of the set of conductors 60 now being out ofalignment with the transverse portions 72 of the set of conductors 68,the resistive output of the impedance-varying means 56 increases. Bymonitoring the size and/or shape change of the bladder by monitoring theresistive output of the sensor 50, the volume of liquid present in thebladder can be readily determined to provide an indication of thefilling state of the bladder.

As in the case of the first embodiment, the sensor of the second tofourth embodiments can also be used for monitoring reflex contractionsof the bladder wall which can then be used by a stimulator to inhibitcontinuance of such contractions to improve continence of the person.

Accordingly, in the case of all four embodiments, the sensor is used forthe purposes of:

(a) monitoring the filling of the bladder, making bladder volumeinformation available to the person, and causing an alarm signal to begenerated when a preset bladder volume is reached; and

(b) monitoring any early or premature (low volume) reflex contractionsof the bladder and using detection of this condition to enableelectrical neuromodulation, thereby inhibiting the reflex contractionsand facilitating continence.

Hence, it is an advantage of the invention, that a sensor is providedwhich facilitates monitoring of a bladder's volume and reflexcontractions of the bladder which can be used in a closed-loopstimulating system for enabling control of the bladder to be effected inpersons who have lost the ability to control functioning of the bladder.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive.

1. An implantable sensor for monitoring the condition of an organ of arecipient's body, the sensor including: a carrier for attachment to anouter surface of the organ; and an impedance-varying means carried bythe carrier, the impedance of the impedance-varying means varying as thecondition of the organ changes to cause a change in output state of theimpedance-varying means.
 2. The sensor of claim 1 in which the carrierincludes an attachment means for attachment to an outer surface of theorgan so that a change in shape of the outer surface causes a change inthe impedance of the impedance-varying means.
 3. The sensor of claim 1in which the carrier comprises a flexible tube having closed ends. 4.The sensor of claim 3 in which the impedance-varying means comprises aconductive material contained in the tube.
 5. The sensor of claim 4 inwhich distortion of the tube causes a change in impedance of theconductive material.
 6. The sensor of claim 5 in which the impedancechange is enhanced by incorporating surface features on an inner wall ofthe tube which increase the influence of the change in the impedance ofthe conductive material as a result of its being distorted.
 7. Thesensor of claim 1 in which the impedance-varying means includes adisplaceable element which is displacably arranged relative to thecarrier, displacement of the element relative to the carrier causing thechange in impedance.
 8. The sensor of claim 7 in which linear motion ofthe displaceable element relative to the carrier causes a change inimpedance which is monitored as a measure of the change in the conditionof the organ.
 9. The sensor of claim 1 in which the carrier is acontainer.
 10. The sensor of claim 9 in which the impedance-varyingmeans is arranged in the container, the impedance-varying meanscomprising a strip of non-conductive material on which a segmentedconductive layer is superimposed.
 11. The sensor of claim 10 in whichthe strip is divided into segments that separate as the change in thecondition of the organ occurs.
 12. The sensor of claim 1 in which thecarrier is in the form of a sealed container in which theimpedance-varying means is received.
 13. The sensor of claim 12 in whichthe impedance-varying means comprises a pair of overlying strips withconductors carried on facing surfaces of the strips, the change incondition of the organ resulting in relative displacement of theconductors on the strips to cause a change in impedance.
 14. The sensorof claim 13 in which the two strips are anchored together at one end.15. The sensor of claim 1 wherein the organ is a bladder.
 16. Animplantable sensor for monitoring the volume of fluid present in arecipient's bladder, the sensor including: a carrier for attachment toan outer surface of the bladder; and an impedance-varying means carriedby the carrier; wherein changes in the outer surface of the bladder asit expands upon receiving fluid are detected as changes in the impedanceof the impedance-varying means.
 17. The sensor of claim 16 whereinchanges in the outer surface of the bladder cause the carrier and theimpedance-varying means to undergo distortion.
 18. The sensor of claim17 wherein the carrier is an enclosed flexible housing.
 19. The sensorof claim 18 wherein the impedance-varying means is contained within theenclosed flexible housing.
 20. The sensor of claim 16 wherein theimpedance-varying means comprises a conductive material.
 21. The sensorof claim 20 wherein distortion of the flexible housing causes a changein impedance of the conductive material.
 22. The sensor of claim 21wherein the impedance change is enhanced by incorporating surfacefeatures on an inner wall of the housing which increase the influence ofthe change in the impedance of the conductive material as a result ofits being distorted.
 23. The sensor of claim 22 wherein the conductivematerial is a conductive fluid.
 24. The sensor of claim 21 wherein theimpedance-varying means comprising a strip of non-conductive material onwhich a segmented conductive layer is superimposed.
 25. The sensor ofclaim 24 in which the strip is divided into segments that separate asthe change in the condition of the bladder occurs.
 26. The sensor ofclaim 21 in which the impedance-varying means comprises a pair ofoverlying strips with conductors carried on facing surfaces of thestrips, the change in the outer surface of the bladder resulting inrelative displacement of the conductors on the strips to cause a changein impedance.
 27. The sensor of claim 26 in which the two strips areanchored together at one end.
 28. An implantable sensor for detectingvariations in shape of an organ of a recipient's body, the sensorcomprising: an elongate flexible housing attachable to an organ of thebody; a conductive material contained within the housing; at least oneelectrode for detecting variations in impedance of the sensor; whereinas the body organ changes shape, the housing also undergoes a change inshape which is detected by the electrode as a variation in the overallimpedance of the conductive material within the housing.
 29. The sensorof claim 28 wherein the housing is stretched on an increase in volume ofthe organ thereby reducing the cross-sectional area of the housing andso increasing the impedance of the conductive material within thehousing.
 30. The sensor of claim 29 wherein the housing comprises aseries of chambers in fluid communication with each other for receivingthe conductive material, each chamber being defined at at least one endby diametrically opposed raised formations in the housing and wherein,on stretching of the housing, the opposed raised formations moverelatively closer to each other thereby further increasing the impedanceof the conductive material within the housing.
 31. The sensor of claim29 wherein the conductive material is a saline solution.
 32. The sensorof claim 28 wherein the organ is a bladder.